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  #1  
Old 06-07-2021, 12:15 PM
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rcarsey rcarsey is offline
 
Join Date: Jul 2012
Location: North Brunswick, NJ
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Default SI-912i-027 (propeller mass-moment measurement)

I did a quick search and didn't see this topic come up yet. Rotax issued SI-912i-027 on 2021-May-28 (also attached here)

It is about ensuring the mass-moment of the propeller is within a certain range for the 912/912i engines. They have you suspend the propeller/hub assembly by two parallel wires. Give the assembly a 10 degree turn, and measure the time it takes for 30 back-forth motions. Then look some numbers up on a chart.

I will assume that the measurements for all of our -12/-12iS aircraft (with factory propellers) would be about the same.. so no need for every owner to test their setup.

But just wanted to start this topic in case anyone has additional information.. or in the off-chance that someone changed to a non-factory propeller/hub.
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RV-12iS (N713) / Completed 12/2020
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  #2  
Old 06-07-2021, 12:23 PM
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rvbuilder2002 rvbuilder2002 is offline
 
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Given that the 912 ULS and iS are both approved to be operated with a constant speed propeller, it is pretty safe to say the the relatively light weight ground adjustable Sensenich propellers we use on the RV-12 would meet the requirement.
Particularly since the blades are so light, but this test has not been done by Van's Aircraft Inc.
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  #3  
Old 06-07-2021, 12:51 PM
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rcarsey rcarsey is offline
 
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True. They had listed a minimum limit as well, which I was more concerned with.

I have no real-life reference as to what these numbers mean, so I can't even make a guess as to whether our setup is within their specs:

Quote:
Min: 1500 kg cm2 (3.559 lb.ft.2)
Max: 6000 kg cm2 (14.238 lb.ft.2)
That said, RV-12 engines/gearboxes have been happy for 10+ years now, so I don't think its an issue.. I don't really want to pull my prop off to do this test if others, or the factory, has already done it or investigated it.
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RV-12iS (N713) / Completed 12/2020
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  #4  
Old 06-08-2021, 01:04 AM
JRichichi JRichichi is offline
 
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Location: League City, TX
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Default Bifilar Moment of Inertia

This method of determining the Moment of Inertia (MOI) of the propeller is call the "Bifilar" method. It works well for long, slender objects like propellers. There's a similar method called the "Trifilar" method. It uses three "wires" and is used for less "slender" shaped objects. The trifilar method would work out great for a three bladed prop. There is a lot of info out there on the web about these two methods. There are even some old NACA papers detailing how this was done on full size aircraft.

One thing that helps is to make sure that the "wire" is a "solid" wire, not a stranded cable. A stranded cable has more dampening in it than a solid wire. They mention using safety wire which should work out great.

On a separate note, we used the trifilar method to measure the MOI on a 1/4 scale splashdown test model for the SpaceX Dragon capsule to to confirm that our CFD analysis of the full size capsule was correct. The CFD analysis of the 1/4 scale model correlated well with the 1/4 scale splashdown tests. This gave us confidence in our CFD model. Our "test pool" was a $200 "kiddie" pool we bought at Walmart. The backdrop we used for the high speed video recordings was a 4'X8' sheet of white tile board (about $10 at the time) and some blue painters tape, laid out on a 12" square grid. The model was dropped from a forklift via some parts we picked up from McMaster-Carr. I had the pleasure of grabbing my scuba gear and underwater video camera to video the test while lying on my back underwater during the test!! The entire project was done by one intern and one technician in about two months. This included making the model, installing the various sensors, test plans, etc. It worked out great. Later on we make a full size trifilar test rig to measure the MOI of the first Dragon capsule. Analysis of the actual splashdown data correlated well with our CFD predictions. It was one of the many things we did at SpaceX that worked well without having to spend a ton of time and money. BTW, we hired the intern who did the model testing after he graduated and he ended up being the engineer in charge for the MOI testing of the actual Dragon capsule!!! Fun times!!!

Jeff
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  #5  
Old 06-08-2021, 05:47 AM
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rv6ejguy rv6ejguy is offline
 
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I did a bifilar suspension on my prop when running a torsional vibration analysis on my engine and gearbox. Video here: https://www.youtube.com/watch?v=D2vtig-E11k
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  #6  
Old 06-08-2021, 05:48 AM
drspud drspud is offline
 
Join Date: Sep 2020
Location: Plant City
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Default MMI

http://www.sensenich.com/wp-content/...2019-04-01.pdf
I refer you to page 8, first section with 2A0R5/R70E. This is the 2 blade propeller.

http://www.sensenich.com/wp-content/...20200204-1.pdf
Page 6. C70MY - this is the three blade propeller
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  #7  
Old 06-08-2021, 12:00 PM
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rcarsey rcarsey is offline
 
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Quote:
Originally Posted by drspud View Post
http://www.sensenich.com/wp-content/...2019-04-01.pdf
I refer you to page 8, first section with 2A0R5/R70E. This is the 2 blade propeller.

http://www.sensenich.com/wp-content/...20200204-1.pdf
Page 6. C70MY - this is the three blade propeller
Nice find.. I didn't even think about consulting the propeller manual.

So the two-bladed R70E is "Less than or equal to 3600 kg-cm^2" and the C70MY is 3700 kg-cm^2.

That makes me a lot more confident that the true number falls within the 1500-6000 range. I'll make a logbook entry and reference the propeller manual information.
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